A standard turbocompound engine includes a turbocharged diesel engine with a power turbine placed downstream of the turbine of the turbocharger. The power turbine recovers exhaust energy and transmits power to the engine output shaft, to which the turbine is coupled by a gear train.
Due to the tremendous amount of braking power required to stop over-the-road trucks, it is constantly a goal to provide improved braking systems. For many internal combustion engine applications, it is highly desirable to operate the engine in a braking mode. One well-known approach, as illustrated in U.S. Pat. No. 3,220,932 which is incorporated herein by reference, has been to convert the engine into a compressor by cutting off the fuel flow and opening the exhaust valve for each cylinder near the end of the compression stroke and to close the exhaust valve shortly thereafter; thus permitting the conversion of kinetic inertial energy of a vehicle to compressed gas energy which may be released to the atmosphere when the cylinder exhaust valves are opened (this form of engine braking is hereinafter referred to as compression braking). When the compression brake is actuated, the engine is prevented from providing positive work to the drive shaft, since gas from the cylinder is expelled through the exhaust valve as the piston completes its compression stroke. Thus, since the fuel flow is cut off and the compressed gas is expelled, no power is transmitted to return the piston on an expansion stroke, but rather, power from the drive shaft is absorbed as the engine acts as a compressor, receiving air from the intake manifold and expelling the compressed gas at the end of each compression stroke.
In compression braking, the power that can be absorbed is related to the air mass flow through the engine, i.e., higher air flow provides higher braking capability. However, during compression braking in a turbocompound engine, the power turbine limits airflow and places a backpressure on the turbocharger turbine. As a result, the pressure drop across the turbocharger turbine is reduced. Since the amount of work produced by the turbocharger in the form of compressed air is related to the pressure drop, when the pressure drop is reduced, the air mass flow provided by the turbocharger is inhibited. In addition, the power turbine contributes power to the engine output shaft which offsets gains in braking power.
Exhaust valving systems which attempt to provide more favorable exhaust pressures for various operating or braking conditions are known. For example, U.S. Pat. No. 4,391,098 to Kosuge discloses a turbocompound engine in which first and second waste gates are provided to control the supercharging pressure and the pressure fed to an auxiliary (power) turbine. The waste gates include pressure sensitive valves which open when the pressure is sufficient to overcome the bias of a spring plus atmospheric pressure. In operation, the first waste gate is opened when the supercharging pressure reaches a predetermined level, and bypasses a portion of the engine exhaust past the turbocharger turbine to the auxiliary turbine. When the engine operates at high speed, high load, the second waste gate is opened by passing at least a portion of the exhaust gases past the auxiliary turbine. However, Kosuge does not provide improved exhaust conditions for increasing the effectiveness of compression braking. The Kosuge arrangement merely provides relief valves to relieve pressures above a predetermined limit.
Japanese Utility Model Laid Open No. 157,941/85 discloses a turbocompound engine in which a power turbine bypass is actuated in response to depression of the accelerator pedal. This arrangement also does not address the problems associated with compression braking in a turbocompound engine.
U.S. Pat. Nos. 4,748,812 and 4,800,726 to Okada et al. disclose turbocompound engines in which the exhaust leading to the auxiliary turbine is blocked during exhaust braking and the auxiliary turbine is utilized as a compressor driven by the engine crankshaft. In this arrangement, the blockage of the exhaust results in high energy back pressure which acts as an engine brake force by increasing the pumping work of the engine. This arrangement is unsuitable for compression braking, since the actuation of the valve upstream of the power turbine closes the exhaust flow passage which diminishes the effectiveness of a compression brake as the inhibited airflow diminishes the ability of the engine to convert kinetic inertial energy of the vehicle to compressed gas energy.